borithan said:

A similar argument could have been made for the jokaero (the actual race, not just their devices) and yet now we have them out on the field.

 Enh, lay off the thunderwolves, the Sons of Russ have been bringing Fenrisian wolves to war for ages, it's perfectly cromulent that if there are bigger, meaner ones than that, they might ride them to war as well.

 I dunno, man. Riding around on giant wolves is kinda silly. I've always much preferred the "space viking" angle on the Sons of Russ over the "wolf wolf wolfy wolf wolf" angle.

Is it just the riding that you don't like? How about if the thunderwolf was a unit upgrade for Fenrisian wolves, or just a follower for a wolf lord?

Also, Norse mythology is chock full o' wolves trying to get loose and chomp everything. If fits more than one might think.

 It's the riding and the fact that EVERYTHING is wolfy. Being accompanied by wolves in battle? Fine, okay. Heck, the Rune Priest in my game has a Fenrisian wolf familiar. Wearing some wolf skins. Yeah, all right. Having practically everything be wolf this and wolf that? I don't like it. I mean, Blood Angels get a pass because not everything is blood this and blood that. The Wolves are really the only ones that do it, and I'd rather have space Viking berserkers that have a wolf theme than Space Furries that Wolf the Wolf Wolf, if you'll pardon the term.

That, and riding a giant wolf is just plain silly.

Gaire said:

That's OK. It isn't actually a wolf. Afterall, there are no wolves on Fenris. Or, at least, there weren't until the Sons of Russ arrived…

Technically, a Thunderwolf is pseudocanine macrofauna, created as a consequence of catastrophic failure of the "Canis Helix" posthuman genetic material. That is, it looks wolf-like, but it's actually far closer to a Space Marine of the Space Wolves in genetics than it is to any species of ancient Terran wolf.

Given that the Space Wolves background has long included the notion that those who fail to overcome their bestial nature during their early trials become savage wolf-like animals (it's mentioned in the original Codex Space Wolves, which was the first Codex published back in the early 90s), I don't think any of this is a huge stretch.

 You… you're kidding, right? Thunderwolves are Space Marines who failed to control the Canis Helix? So the Space Wolves are supposedly riding actual furry/werewolf failed Marines? I'm pretty sure that link is never given in GW published work…

A lot of this is fan theory, but it makes sense. Okay, so Fenris was settled in the 20something millenium by 'normal' humans, right? During the great expansions, before the Imperium of Man. Cut off from Earth during the Age of Strife by the Warp Storms. Now Fenris was made habitable via terraforming, but they also did some genetic manipulation of the settlers. Fenris being a death world, such modifications were required so people could survive there. Over time, that genetic manipulation led to mutations, eventually creating the Fenrisian wolves. Fast forward to, oh, M29 or M30. Primarchs and Space Marines created. Russ arrives on Fenris. Some theories say that something about Fenris- radiation, maybe, I dunno- altered him somewhat, making him more bestial and lupine. The Canis Helix is later codified as a mutation of the Space Wolves' gene-seed, which results in the Wulfen and possibly the Thunderwolves.

A lot of the support comes from the novel "A Thousand Sons", in which Magnus the Red utters the phrase "There are no wolves on Fenris" several times. I'm not exactly well-versed in this theory, so anyone can feel free to correct me on any point.

EDIT: It should be noted that neither standard Fenrisian wolves nor Blackmanes could have originated from mutated Space Marines, as Russ was raised by a Blackmane she-wolf alongside Freki and Geri.

Wolf-LIKE. I always read that as being like most representations of Werewolves crossed with Ogryns or something. The Thunderwolf cavalrys' mounts are purely wolf in anatomy.

I DON'T KNOW WHAT TO THINK

I think Space Wolves are totally the kind of guys who would go in to battle riding Giant Wolves. It seems to me to be that that is very in keeping with how they roll. 

Don't know how I feel about Thunderwolves being descended from failed (too successful?)  Marines. It has a pleasing kind of weirdness, but I don't know how cool it is to ride your mutated bretheren.

 Well, I've said my piece, but that's just my opinion. So no worries.

Gaire said:

It's dealt with in both A Thousand Sons and Prospero Burns, which both cover the events surrounding the Council of Nikea and the Scouring of Prospero, though from differing perspectives. A further nod is given in Deliverance Lost, which mentions amongst other little facts about the Primarch genetics that Leman Russ was engineered with canine genes from the start - which also suggested that the differences between Primarchs may have been at least partly deliberate rather than purely the consequence of their scattering through the Warp.

As for the Blackmanes and Freki and Geri… all we know of the history of Leman Russ is legend. Who knows what truth that legend is built upon?

Hmmmm… heh cool, they got the rights to publish Blood Ravens crunch from Relic.  Raptors sound like Catachans with geneseeds, which is kind of neat I guess.  And then there's the space marine pirates the Space Sharks (I will never be able to spell the latin version without it being right in front of me,) am I wrong to picture one of those guys in an RT's retinue?  Okay FFG, you win, you finally convinced me that maybe, just maybe, Space Marines MIGHT be as cool as the Guard. :P

 I like how the majority of Chapters from the Badab War were included, that's great!

However!!!

There are no Mantis Warriors or Executioners. Mantis Warriors are particularly interesting.

Mantis Warriors are quite hard for me to picture, in fact, stat-wise. Bonus to Agility and maybe Perception, Intelligence or Willpower.

Executioners would likely have a similar write-up to the Subjugators.

I understand correctly, how much more did they put in for the Flesh Tearers then was Rites of Battle?